High-speed verifiable semiconductor memory device

ABSTRACT

A memory cell stores several data using n (n: natural number more than 1) threshold voltages. A voltage supply circuit supplies a predetermined voltage to a gate of the memory cell in a verify operation of verifying whether or not the memory cell reaches a predetermined threshold voltage. A detection circuit connected to one terminal of the memory cell charges one terminal of the memory cell to a predetermined potential. The detection circuit detects the voltage of one terminal of the memory cell based on a first detection timing, and further, detects the voltage of one terminal of the memory cell based on a second detection timing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/297,467, filed Dec. 9, 2005, now U.S. Pat. No. 7,196,933 and is basedupon and claims the benefit of priority from prior Japanese PatentApplication No. 2004-359029, filed Dec. 10, 2004, the entire contents ofeach of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device, which iscapable of storing binary or more data in one memory cell.

2. Description of the Related Art

There has been proposed a NAND flash memory using an EEPROM, that is, anon-volatile semiconductor memory device capable of storing multi-valueddata in one memory cell (e.g., see JPN. PAT. APPLN. KOKAI PublicationNo. 2000-195280).

The NAND flash memory is configured in a manner that all or half ofseveral memory cells arrayed in a row direction are each connected tothe corresponding latch circuit via a bit line. The latch circuit holdsdata in data write and read. The foregoing all or half of cells arrayedin the row direction collectively write and read data (e.g., see JPN.PAT. APPLN. KOKAI Publication No. 2003-196988).

Quick Pass Write (QPW) has been proposed as a method of preventing adata write time with respect to memory cell from increasing andnarrowing a threshold voltage distribution width after data write.According to the QPW, a bit line is charged while a potential (voltage)of word line is set to a level lower than a normal verify level.Thereafter, a select gate is set to a high level, and then, thepre-charged bit line is discharged. In non-write cell and writecontinuing cell, which are far from a write threshold voltage, the bitline potential becomes a low level because current is carried. On theother hand, in a write continuing cell, which is close to the writethreshold voltage and a write completed cell, the bit line is intactlykept high. At that time, the bit line potential is detected (first timeverify).

Then, the word line potential is set to a normal word line level. Bydoing so, in the write continuing cell, the bit line becomes low.Therefore, in the write completed cell only, the bit line potentialbecomes high. At that time, the bit line potential is detected (secondtime verify).

According to the result of the second time verify, a cell becoming highis a write completed cell. The write completed cell is regarded as anon-write cell in the next program loop, and therefore, it sets the bitline to a power supply voltage Vdd, and does not carry out a writeoperation. On the other hand, according to the result of the second timeverify, a cell becoming low is a write incomplete cell. The writeincomplete cell carries out a write operation in the next program loop.However, according to the result of the first time verify, a cellbecoming high is a cell, which is close to the threshold voltage (normalverify level). For this reason, the cell sets the bit line to anintermediate potential (e.g., 0.75V), and carries out a write operationin a state of reducing a write speed. On the other hand, according tothe result of the first time verify, a cell becoming low sets the bitline to a ground voltage Vss, and carries out a write operation.

In this manner, program and verify are repeated until all write cellspass the second time verify. Therefore, the write speed of the cellsclosing to a normal verify potential becomes late; as a result,threshold voltage distribution is narrowed.

However, the first time verify operation is made in a state that theword line potential is set to a level lower than a normal verify level.Thereafter, the second time verify operation must be made in a statethat the word line potential is set as the normal write verify level.The word line has a large capacity; for this reason, time is taken tostep up the word line potential. As a result, there is a problem thatthe verify time increases. Therefore, it is desired to provide asemiconductor memory device capable of preventing the verify time fromincreasing.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda semiconductor memory device comprising: a memory cell storing datausing n threshold voltage (n: natural number more than 1); a voltagesupply circuit supplying a predetermined voltage to a gate of the memorycell in a verify operation of verifying whether or not the memory cellreaches a predetermined threshold voltage; and a detection circuitconnected to one terminal of the memory cell, the detection circuitcharging one terminal of the memory cell to a predetermined potential inthe verify operation, detecting a voltage of one terminal of the memorycell at a first detection timing, and detecting a voltage of oneterminal of the memory cell at a second detection timing.

According to a second aspect of the present invention, there is provideda semiconductor memory device comprising: a memory cell storing datausing n threshold voltage (n: natural number more than 1); a first datastorage circuit connected to one terminal of the memory cell, andstoring first or second logic level data inputted externally; a seconddata storage circuit connected to one terminal of the memory cell, andstoring the first or second logic level data; and a control circuitconnected to the first and second data storage circuits, the controlcircuit carrying out the following:

a first operation of stepping up the threshold voltage of the memorycell when the first and second data storage circuits are a first logiclevel; a second operation of stepping up the threshold voltage of thememory cell in a range of the threshold voltage less than the firstoperation when the second data storage circuits is a second logic level;an operation of charging one terminal of the memory cell while applyinga first voltage to a gate of the memory cell in a verify operation ofholding the threshold voltage of the memory cell without changing it andverifying whether or not the memory cell reaches a predeterminedthreshold voltage when the first data storage circuits is a second logiclevel; and the first operation in a manner of changing the logic levelof the second data storage circuit to a second logic level when thevoltage of one terminal of the memory cell is more than a firstdetection level in a first step, and after predetermined time elapses,changing the logic level of the first data storage circuit to a secondlogic level when the voltage of one terminal of the memory cell is morethan a second detection level in a second step.

According to a third aspect of the present invention, there is provideda semiconductor memory device comprising: a memory storing several datausing n threshold voltage (n: natural number more than 1); a first datastorage circuit connected to one terminal of the memory cell, andstoring first or second logic level data input externally; and a controlcircuit connected to the first data storage circuit, the control circuitcarrying out a first operation of stepping up the threshold voltage ofthe memory cell when the first data storage circuits is a first logiclevel, charging one terminal of the memory cell when the first datastorage circuit is a first logic level while applying a first voltage toa gate of the memory cell without making no charge operation when it issecond level in a verify operation of holding the threshold voltage ofthe memory cell without changing it and verifying whether or not thememory cell reaches a predetermined threshold voltage when the firstdata storage circuits is a second logic level, and making no firstoperation in a manner of changing the logic level of the first datastorage circuit to a second level when the voltage of one terminal ofthe memory cell is more than a first detection level.

According to a fourth aspect of the present invention, there is provideda program method of a semiconductor memory device comprising: carryingout a write operation with respect to a memory cell storing data usingdifferent threshold voltage, and making no change of a threshold voltagein a next write operation when the memory cell reaches a predeterminedfirst threshold voltage in a verify operation of and verifying whetheror not the memory cell reaches a predetermined first threshold voltage;and carrying out an i (i: natural number more than 3) write operation inthe next write operation when the memory cell reaches a voltage lowerthan the predetermined first threshold voltage, that is, i thresholdvoltage (i: natural number more than 3) (first threshold voltage>secondthreshold voltage>third threshold voltage . . . >i threshold voltage),and controlling the threshold voltage in the i write operation so that achange becomes small in the order of first threshold voltage<thirdthreshold voltage<fourth threshold voltage< . . . <k threshold voltage,and repeating the write operation and the verify operation until thememory cell reaches the predetermined first threshold voltage.

According to a fifth aspect of the present invention, there is provideda semiconductor memory device comprising: a memory cell array havingseveral memory cells arrayed like a matrix, which are connected to aword line and a bit line and store n value (n: natural number more than3); and a control circuit controlling each potential of the word lineand the bit line in accordance with input data, and writing data in thememory cell, the control circuit carrying out write using k value (k<=n)in the write operation, charging the bit line, and thereafter, changingthe word line potential m times to verify whether or not the memory cellreaches a normal m threshold value (m<=k), charging the bit line in jvalue (j<=n) data read, and thereafter, changing a voltage supplied tothe word line m times same as the verify operation to carry out a readoperation.

According to a sixth aspect of the present invention, there is provideda semiconductor memory device comprising: a memory cell array havingseveral memory cells arrayed like a matrix, which are connected to aword line and a bit line and store n value (n: natural number more than3); and a control circuit controlling each potential of the word lineand the bit line in accordance with input data, and writing data in thememory cell, the control circuit carrying out write using k value (k<=n)in the write operation, and charging the bit line, thereafter, changingthe word line potential m times to verify whether or not the memory cellreaches a normal m threshold value (m<=k), charging the bit line again,thereafter, changing the word line potential m times to verify whetheror not the memory cell reaches a normal m threshold value (m<=k), and inthe next write operation, making a write speed late when the memory cellreaches the normal m threshold value while carrying out no writeoperation when it reaches the normal m threshold value.

According to a seventh aspect of the present invention, there isprovided a semiconductor memory device comprising: a memory cell arraystoring n value (n: natural number more than 2), and having at least onefirst memory cell arrayed like matrix and at least one second memorycell selected simultaneously with the first memory cell, the memory cellarray outputting data of the first memory cell when a logic level of thesecond memory cell is a first logic level, and outputting output data ofthe first memory cell as a fixed value when the logic level of thesecond memory cell is a second logic level.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1A is a waveform chart showing word line and bit line potentialsaccording to a first embodiment;

FIG. 1B is a waveform chart showing a program verify operation ofchanging a word line potential;

FIG. 1C is a waveform chart showing each potential of word line WL andbit line BL in a read operation;

FIG. 2 corresponds to FIG. 1A, and is a view showing Vg-Id (gatevoltage-drain current) characteristic of cell;

FIG. 3A is a view showing program step count and a change of thresholdvoltage in the first embodiment;

FIG. 3B is a view showing program step count and a change of thresholdvoltage in QPW;

FIG. 3C is a view showing a modification example of FIG. 3A;

FIG. 4 is a block diagram showing the configuration of a semiconductormemory device according to a first embodiment;

FIG. 5 is a circuit diagram showing each configuration of a memory cellarray 1 and a bit line control circuit 2 shown in FIG. 4;

FIG. 6A and FIG. 6B are cross-sectional views showing each structure ofmemory cell and select transistor;

FIG. 7 is a cross-section view showing the structure of one NAND cell ofthe memory cell array;

FIG. 8 is a circuit diagram showing the configuration of a data storagecircuit shown in FIG. 5;

FIG. 9 is a view to explain the relationship between data and thresholdvoltage in a memory cell according to the first embodiment;

FIG. 10 is a flowchart to explain a first page program operation;

FIG. 11A, FIG. 11B and FIG. 11C are views showing the relationshipbetween data stored in a data cache and write cells;

FIG. 12 shows a modification example, and is a view showing Vg-Idcharacteristic;

FIG. 13 is a flowchart to explain an operation according to amodification example of the first embodiment;

FIG. 14 is a flowchart to explain an erase operation a modificationexample of the first embodiment;

FIG. 15 is a flowchart to explain a program operation a modificationexample of the first embodiment;

FIG. 16 is a view showing the content of data cache in the programoperation shown in FIG. 15;

FIG. 17 is a waveform chart showing an operation of a third embodiment;

FIG. 18 is a circuit diagram showing the configuration of a data storagecircuit applied to a fourth embodiment;

FIG. 19 is a flowchart to explain an operation according to a fourthembodiment;

FIG. 20 is a view showing the operation of the fourth embodiment;

FIG. 21 is a view showing a program verify read operation;

FIG. 22 is a view showing a program verify read operation according to afifth embodiment;

FIG. 23 is a circuit diagram showing the configuration of a data storagecircuit applied to a fifth embodiment;

FIG. 24 is a view showing Log (I)-V characteristic;

FIG. 25 is a waveform chart when a potential change of word line in aread operation according to a sixth embodiment is aligned with that in averify operation;

FIG. 26A, FIG. 26B and FIG. 26C are views to explain an operation ofstoring quaternary (four-valued) data according to a seventh embodiment;

FIG. 27 is a flowchart to explain a first page program operation;

FIG. 28 is a flowchart to explain a second page program operation;

FIG. 29 is a flowchart to explain a second page program operationaccording to a seventh embodiment;

FIG. 30 is a waveform chart showing a second page program operation;

FIG. 31 is a flowchart to explain a first page read operation ofchanging a verify level;

FIG. 32 is a flowchart to explain a second page read operation when theverify level is changed and unchanged;

FIG. 33 is a flowchart to explain a first page read operation accordingto a seventh embodiment;

FIG. 34 is a waveform chart showing the first page read operationaccording;

FIG. 35 is a waveform chart showing a modification example of theseventh embodiment;

FIG. 36 is a waveform chart showing a first page read operationaccording to an eighth embodiment;

FIG. 37 is a waveform chart showing a second page read operationaccording to an eighth embodiment; and

FIG. 38 is a waveform chart showing word line and bit line potentialsaccording to a modification example of the fifth to seventh embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawings.

First Embodiment

The principle of the first embodiment will be explained below.

FIG. 1A shows word line and bit line potentials in the first embodiment,and FIG. 2 shows Vg-Id (gate voltage-drain current) characteristic ofeach cell. As shown in FIG. 1A, according to the first embodiment, theword line potential is set to a normal write verify level (AV), andthen, the difference of discharge speed of the bit line is detected,thereby carrying out a first time verify. Thereafter, a normal verifyoperation, that is, second time verify operation is carried out withoutchanging the word line potential. On the contrary, FIG. 1B shows aprogram verify operation of changing the word line potential.

As illustrated in FIG. 1A, the bit line is charged while the potentialof a word line WL is set to a normal verify level (AV). Thereafter, aselect gate is set high and a pre-charged bit lone BL is discharged. Ina non-write cell (shown by (1) in FIG. 1A) and a write continuing cell(shown by (2) in FIG. 1A), which is far from a write threshold voltage,the bit line potential becomes low because current is carried. On theother hand, in a write continuing cell (shown by (3) in FIG. 1A), whichis close to the write threshold voltage and a write completed cell(shown by (4) in FIG. 1A), the bit line potential is intactly kept high.At that time, the bit line potential is detected (first time verifyVF1). In this case, the bit line potential is detected using a detectionlevel SLH higher than a normal bit line potential.

The discharge of the bit line is continued, and thereby, the bit linepotential becomes low in the write continuing cell (shown by (3) in FIG.1A). Therefore, the bit line potential becomes low in only writecompleted cell (shown by (4) in FIG. 1A). At that time, the bit linepotential is detected (second time verify VF2). The detection level is anormal detection level SL.

According to the result of the second time verify, a cell becoming highis a write completed cell, likewise the foregoing QPW. Thus, in the nextprogram loop, the write completed cell is regarded as a non-write cell,and then, the bit line is supplied with power supply voltage Vdd not tocarry out a write operation. A write incomplete cell according to theresult of the second time verify carries out a write operation in thenext program loop. However, if the write incomplete cell is a cellbecoming high according to the result of the first time verify, the bitline is supplied with an intermediate voltage (e.g., 0.75V) to carry outa write operation at a reduced write speed. Moreover, if the cell is acell becoming low according to the result of the first time verify, thebit line is supplied with a ground voltage Vss to carry out a writeoperation.

In the manner as described above, program and verify are repeated untilall write cells pass the second time verify. Therefore, the write speedof the cells closing to a normal verify potential becomes late; as aresult, threshold voltage distribution is narrowed.

According to the first embodiment, there is no need of changing the wordline potential in the program verify operation; therefore, high-speedwrite is provided.

FIG. 1C shows each waveform of word line WL, bit line BL and select gateSG in a read operation. As is evident from FIG. 1C, a normal verify readoperation and the read operation become the same operation excludingword line potential. In other words, the bit line discharge time becomesequal in the foregoing normal verify read operation and read operation;therefore, the displacement of threshold voltage is reduced. Thus, datawritten in the memory cell is securely read. Therefore, this serves toreduce a margin between the normal verify read operation and the readoperation.

FIG. 3A shows program step count and a change of threshold voltage inthe first embodiment. FIG. 3B shows program step count and a change ofthreshold voltage in QPW. In the case shown in FIG. 3B, if thresholdvoltage exceeds a verify level (AVL) lower than a normal verify level(AV), the bit line is supplied with an intermediate potential (e.g.,0.75V) to reduce a write speed in the next write operation.

On the other hand, in the first embodiment shown in FIG. 3A, if thethreshold voltage is lower than a normal verify level (AV), thethreshold voltage of memory cell is detected at a cell current dischargespeed without changing the word line level. For this reason, S-factor(cell current to word line potential) is variable due to back pattern(characteristic changes resulting from other cell write to NAND cell).Therefore, detection level is not uniform, and a write speed is reduced.

In this case, as depicted in FIG. 3C, the intermediate potentialsupplied to the bit line is set to an intermediate voltage (e.g., 0.4V)lower than a normal case to slightly enhance the write speed.

The first embodiment will be explained in detail.

FIG. 4 shows the configuration of a semiconductor memory deviceaccording to a first embodiment, that is, NAND flash memory storingbinary data.

A memory cell array 1 comprises a plurality of electrically datarewritable memory cells, which are arrayed like a matrix. The memorycells each includes several bit lines, several word lines and sharedsource line, and comprises an EEPROM cell. The memory cell array 1 isconnected with bit line control circuit 2 for controlling bit lines andword line control circuit 6.

The bit line control circuit 2 includes several data storage circuitsand flag data storage circuits described later. The bit line controlcircuit 2 carries out the following operations. Specifically, thecontrol circuit 2 reads data of a memory cell of the memory cell array 1via the bit line. The control circuit 2 detects a state of the memorycell of the memory cell array 1 via the bit line. The control circuit 2applies a write control voltage to the memory cell of the memory cellarray 1 via the bit line to make write to the memory cell. Moreover, thebit line control circuit 2 is connected with column decoder 3, datainput-output buffer 4. The column decoder 3 selects a data storagecircuit of the bit line control circuit 2. Memory cell data read to thedata storage circuit is externally output from a data input-outputterminal 5 via the data input-output buffer 4.

Write data input to the data input-output terminal 5 from the outside isinput to the data storage circuit selected by the column decoder 3 viathe data input-output buffer 4.

The word line control circuit 6 is connected to the memory cell array 1.The word line control circuit 6 selects a word line of the memory cellarray 1, and applies a voltage required for read, write or erase to theselected word line.

The foregoing memory cell array 1, bit line control circuit 2, columndecoder 3, data input-output buffer 4 and word line control circuit 6are connected to a control signal and control voltage generator circuit7. The control signal and control voltage generator circuit 7 controlsthe foregoing components. The control signal and control voltagegenerator circuit 7 is connected to a control signal input terminal 8,and controlled according a control signal inputted via the terminal 8from the outside.

The foregoing bit line control circuit 2, column decoder 3, word linecontrol circuit 6 and control signal and control voltage generatorcircuit 7 form write circuit and read circuit.

FIG. 5 shows each configuration of the memory cell array 1 and the bitline control circuit 2 shown in FIG. 4. Several NAND cells are arrayedin the memory cell array 1. One NAND cell is composed ofserial-connected 16 memory cells MC each comprising EEPROM, select gatesS1 and S2. The select gate S2 is connected to a bit line BL0; on theother hand, the select gate S1 is connected to a source line SRC. Thecontrol gate of each row-arrayed memory cell MC is connected in commonto word lines WL0 to WL13, WL14 and WL15. The select gate S2 isconnected in common to a select line SGD; on the other hand, the selectgate S1 is connected in common to a select line SGS.

The bit line control circuit 2 has several data storage circuits 10. Thedata storage circuits 10 are respectively connected with a pair of bitlines (BL0, BL1), (BL2, BL3) . . . (BLi, BLi+1) and (BL8044, BL8045).

The memory cell array 1 includes several blocks shown by the brokenline. Each block is composed of several NAND cells, and data is erasedat a unit of block. The erase operation is simultaneously carried outwith respect to two bit lines connected to the data storage circuit 10.

Several memory cells (surrounded by the broken line), which are arrayedon every other bit line and connected to one word line, form one sector.Data is written and read every sector.

One of two bit lines (BLi, BLi+1) connected t the data storage circuit10 is selected in read operation, program verify operation and programoperation. In this case, one bit line is selected in accordance withexternally supplied address signals (YA1, YA2 . . . YAi . . . YA4023).Moreover, one word line is selected in accordance with an externaladdress.

FIGS. 6A, 6B and FIG. 7 are cross-sectional views showing each structureof memory cell and select transistor.

FIG. 6A and FIG. 6B are cross-sectional views showing each structure ofmemory cell and select transistor. FIG. 6A shows a memory cell. Asubstrate 41 is formed with an n-type diffusion layer 42 as asource/drain of the memory cell. A floating gate (FG) 44 is formed abovethe substrate 41 via a gate insulating film 43. A control gate (CG) 46is formed above the floating gate 44 via an insulating film 45. FIG. 6Bshows a select gate. A substrate 41 is formed with an n-type diffusionlayer 47 as a source/drain. A control gate 49 is formed above thesubstrate 41 via a gate insulating film 48.

FIG. 7 shows the cross-section of one NAND cell of the memory cellarray. As shown in FIG. 7, one NAND cell has 16 memory cells having thestructure shown in FIG. 6A, which are serial-connected. The source/drainside of the NAND cell is provided with select gates S1 and S2 having thestructure shown in FIG. 6B.

FIG. 8 is a circuit diagram showing the configuration of the datastorage circuit 10 shown in FIG. 5.

The data storage circuit 10 has primary data cache (PDC), dynamic datacache (DDC) and temporary data cache (TDC). The foregoing PDC and DDChold input data in a write operation, hold read data in a readoperation, temporarily hold data in a verify operation, and are used tooperate internal data when storing multi-valued data. The TDC amplifiesbit line data in a data read operation while temporarily holds it, andis used to operate internal data when storing multi-valued data.

An input-output data line IO is connected to a node N1 b of the PDC viaa column select transistor 61 e. An input-output data line IOn isconnected to a node N1 a of the PDC via a column select transistor 61 f.Each gate of these transistors 61 e and 61 f is supplied with a columnselect signal CSLi. A transistor 61 d is connected between the node N1 aand ground. The gate of the transistor 61 d is supplied with a signalPRST.

The PDC is composed of clocked inverter circuits 61 i, 61 j andtransistor 61 k. The transistor 61 k is connected between imputerterminals of clocked inverter circuits 61 i and 61 j. The gate of thetransistor 61 k is supplied with a signal EQ1. The node N1 b of the PDCis connected to the gate of a transistor 61 l. One terminal of thecurrent path of the transistor 61 l is grounded via a transistor 61 m.The gate of the transistor 61 m is supplied with a signal CHK1. Theother terminal of the current path of the transistor 61 m is suppliedwith a signal COMi. The signal COMi is a signal, which is common to alldata storage circuits 10 and show whether the verify operation of alldata storage circuits 10 is completed. Specifically, when the verifyoperation is completed, the node N1 b of the PDC becomes low, asdescribed later. In this state, when the signal CHK1 is set high, thesignal COMi becomes high if the verify operation is completed.

The node N1 a is further connected to a node N3 via a transistor 61 h.The gate of the transistor 61 h is supplied with a signal BLC1. Theforegoing TDC is connected between the node N3 and a ground. The TDCcomprises a MOS capacitor 61 p, for example. The node N3 is connectedwith the DDC via a transistor 61 q. The gate of the transistor 61 q issupplied with a signal REG.

The DDC is composed of transistors 61 r and 61 s. One terminal of thecurrent path of the transistor 61 r is supplied with a signal VREG whilethe other terminal thereof is connected to the current path of thetransistor 61 q. The gate of the transistor 61 r is connected to thenode N1 a of the PDC via the transistor 61 s. The gate of the transistor61 s is supplied with a signal DTG.

The N3 is connected with one terminal of each current path oftransistors 61 t and 61 u. The other terminal of the current path of thetransistor 61 u is supplied with a signal VPRE while the gate thereof issupplied with a signal BLPRE. The gate of transistor 61 t is suppliedwith a signal BLCLAMP. The other terminal of the current path of thetransistor 61 t is connected to one end of a bit line BLo via atransistor 61 v while one terminal thereof is connected to one end of abit line BLe via a transistor 61 w. The other end of the bit line BLo isconnected to one terminal of the current path of a transistor 61 x. Thegate of the transistor 61 x is supplied with a signal BlASo. The otherend of the bit line BLe is connected to one terminal of the current pathof a transistor 61 y. The gate of the transistor 61 y is supplied with asignal BlASe. The other terminal of each current path of thesetransistors 61 x and 61 y is supplied with a signal BLCRL. Thetransistors 61 x and 61 y are complementarily powered on in accordancewith signals BlASo, BlASe to supply a potential of the signal BLCRL to anon-select bit line.

The foregoing signal and voltage are generated from the control signaland control voltage generator circuit 7 shown in FIG. 4. The followingoperation is controlled based on control of the control signal andcontrol voltage generator circuit 7.

The present memory is a binary memory, and capable of storing 1-bit datain one cell, for example.

FIG. 9 shows the relationship between data and threshold voltage inmemory cell. When the memory carries out an erase operation, thethreshold voltage of the memory cell becomes negative, and then, writeand read data become “1”. The threshold voltage is stepped up from thepreceding state according to a write operation, and thereby, write andread data become “0”.

(Program and Program Verify)

FIG. 10 shows a first page program operation. The program operation willbe explained below with reference to FIG. 10 and FIG. 8.

(Data Read) (S11)

According to the program operation, an address is fist designated toselect one sector shown in FIG. 5. Data to be written is input from theoutside, and stored in each PDC of all data storage circuits 10. If data“1” (do not carry out) is supplied from the outside, the node N1 a ofthe PDC becomes high. On the other hand, if data “0” (carry out write)is supplied, the node N1 a of the PDC becomes low. In the followingdescription, PDC data is given as a potential of the node N1 a; on theother hand, DDC data is given as a potential of the node N2.

(Setting of Data Cache) (S12)

When a write command is supplied, the signal DTG instantaneously becomeshigh, and then, PDC data is copied into the DDC.

(Program) (S13)

When the potential of the signal BLC is set to Vdd+Vth (Vth: thresholdvoltage of N channel MOS transistor), the bit line becomes power supplyvoltage Vdd if data “1” (do not carry out write) is stored in the PDC.If data “0” (carry out write) is stored therein, the bit line becomesground potential Vss. Write must not be carried out with respect tocells connected to the selected word line and having non-select page,that is, non-select bit line. Thus, bit lines connected to these cellsare set to voltage Vdd. Here, the select line SG1 of the selected blockis set to Vdd, and Vpgm (e.g., 20V) is supplied to a select word linewhile Vpass (e.g., 10V) is supplied to a non-select word line. In thiscase, the channel of the cell becomes voltage Vss, and the word linebecomes voltage Vpgm; therefore, write is carried out. On the otherhand, if the bit line becomes Vdd, the channel of the cell steps upVpgm, and not Vss; therefore, it becomes Vpgm/2 in coupling. For thisreason, program is not executed.

The signal BLC is set low while the signal REG is set to an intermediatepotential (e.g., 0.75+Vth), and thereby, the bit line becomes theintermediate potential only when PDC is low while DDC is high. Thus, thewrite speed becomes later than normal write; as a result, write isslightly carried out. On the other hand, if PDC is L level while DDC islow, the potential of the bit line is kept at Vss. Thus, write isnormally carried out. Moreover, if PDC is high, write is not carriedout. In a first time program operation (before a verify operation ismade), PDC=DDC is given. Therefore, the bit line does not become anintermediate potential. As depicted in FIG. 9, if data “0” is written inthe memory cell, memory cell data is set to “1”. If data “1” is writtenin the memory cell, memory cell data is set to “0” as it is unchanged.

(Program Verify) (S14)

According to a program verify operation, a select word line of theselected block is supplied with verify potential AV (e.g., 0.8V) highthan read potential AR (e.g., 0V or 0.2V). Non-select word line andselect line SG1 are supplied with Vread. Simultaneously, the bit line ispre-charged under condition that signal VPRE=Vdd, signal BLPRE=Vdd,signal BLCLAMP=Vpre+Vth (e.g., 0.7V+Vth).

The source-side select line SGS of the cell is set high. As shown inFIG. 1 a and FIG. 2, in non-write cell (1) and write continuing cell (2)far from write threshold voltage, the bit line potential becomes lowbecause current is carried. However, in write continuing cell (3) closeto write threshold voltage, the cell current is a little; for thisreason, the bit line potential merely slightly steps down. In the writecompleted cell (4), the bit line potential is kept high.

In the data storage circuit 10, signal VPRE=Vdd and signal BLPRE=Vdd+Vthare given to pre-charge the TDC to Vdd. Thereafter, when signalBLCLAMP=Vsen0+Vth (e.g., 0.65V+Vth) is given, TDC becomes high if thebit line potential is higher than 0.65V. On the other hand, TDC becomeslow if the bit line potential is lower than 0.65V. In other words, theTDC becomes low in the cases (1) and (2); on the other hand, it becomeshigh in the cases (3) and (4) (first time verify VF1).

The bit line detection potential Vsen0 may be set to a level higher thana normal bit line detection potential Vsen (e.g., 0.45V) or to thepotential same as that.

When signal VPRE=Vdd and signal REG=Vdd are given, if DDC=high, that is,it becomes high in write non-select or the first time program verify,TDC forcedly becomes high. Thereafter, signal DTG=Vdd+Vth is given tocopy PDC data into DDC. Thereafter, signal BLC=Vdd+Vth is given tocapture the potential of the TDC in the PDC. PDC=high is given in writenon-select cell or cells (3) and (4). On the other hand, PDC=low isgiven in cells (1) and (2).

The discharge of the bit line is continued, and thereby, the bit line ofthe write continuing cell (3) is discharged; therefore, it becomes low.As a result, the potential of the bit line becomes high in the writecompleted cell only.

In the data storage circuit, signal VPRE=Vdd and signal BLPRE=Vdd+Vthare given to pre-charge the TDC to Vdd. Thereafter, signal BLCLAMP isset to Vsen+Vth (e.g., 0.45V+Vth). If the bit line potential is higherthan 0.45V, the TDC becomes high. On the other hand, if the bit linepotential is lower than 0.45V, the TDC becomes low. In other words, theTDC becomes low in the cases (cells) (1), (2) and (3), and becomes highin the case (cell (4) (second time verify VF2).

Moreover, when signal VPRE=Vdd and signal REG=Vdd are given, ifDDC=high, that is, it becomes high in write non-select or the secondtime program verify (program is completed), TDC forcedly becomes high.Thereafter, signal DTG=Vdd+Vth is given to copy PDC data into DDC.Thereafter, signal BLC=Vdd+Vth is given to capture the potential of theTDC in the PDC. PDC=high is given in write non-select cell or writecompleted cell. On the other hand, PDC=low is given in write incompletecell.

FIG. 11A, FIG. 11B and FIG. 11C show the relationship between datastored in data cache and write cell.

Thereafter, program and verify operations are repeated until data ofeach PDC of all data storage circuits becomes high. If DDC is highduring the program operation, the bit line is supplied with anintermediate potential to make write speed late, and thereby, thethreshold voltage distribution width of cell is reduced.

(Read)

According to a read operation, a read potential AR (e.g., 0V or 0.2V) issupplied to a select word line of the selected block shown in FIG. 5 tosupply a voltage Vread to non-select word line and select line SGD.Simultaneously, in the data storage circuit 10 shown in FIG. 8, signalVPRE=Vdd, signal BLPRE=Vdd and signal BLCLAMP=Vpre+Vth (e.g., 0.7V+Vth)are given to pre-charge the bit line.

The source-side select line SGS of the cell is set high.

FIG. 1C shows each potential of word line WL and bit line BL in a readoperation. In the read operation, it is possible to carry out the secondtime verify only made in the program verify operation. However, the sameoperation as the program verify operation is carried out, therebyreducing margin between the program verify operation and the readoperation. Thus, in the data storage circuit 10, signal VPRE=Vdd andsignal BLPRE=Vdd+Vth are given to pre-charge the TDC to Vdd, like theprogram verify operation. Thereafter, signal BLCLAMP=Vpre+Vth (e.g.,0.7V+Vth) is given, and then, the discharge of the bit line iscontinued. Thereafter, in the data storage circuit, signal VPRE=Vdd,signal BLPRE=Vdd+Vth are again given to pre-charge the TDC to Vdd. Bydoing so, signal BLCLAMP=Vsen+Vth e.g., 0.45V+Vth) is given. If the bitline potential is higher than 0.45V, TDC=high is obtained. If the bitline potential is lower than 0.45V, TDC=low is obtained. Thereafter,signal BLC=Vdd+Vth is given to capture the TDC potential in the PDC. Asseen from FIG. 9, if write and read cell data are “1”, PDC=low is given;on the other hand, if the data is “1”, PDC=high is given. However, thesedata are inverted, and thereafter, output outside.

(Erase)

According to an erase operation, erase is carried out every one blockshown in FIG. 5. When erase is carried out, write and read cell databecomes “1” as shown in FIG. 9.

(Modification Example of First Embodiment)

(Change of First Time Verify Detection Potential (Vsen0) or DetectionTiming)

Program and erase operations are repeated, and thereby, current flowingthrough the cell is reduced, and the gradient of Vg-Id characteristicbecomes small as shown by (2) in FIG. 12. For this reason, in a writecontinuing cell (2) far from threshold voltage, the case where the PDCbecomes high in the first time verify increases. When the PDC becomeshigh, an intermediate potential is supplied in the next program; forthis reason, the write speed becomes late. As a result, the followingproblem arises. Specifically, the cell does not readily reach a normal(predetermined) threshold voltage; for this reason, the program verifyloop count increases until it reaches the normal threshold voltage.

In view of the foregoing circumstances, the control signal and controlvoltage generator circuit 7 shown in FIG. 4 is provided with a counter.The counter counts loop every several page or block writes. If thecounted number of loops is more than a preset value, the level of Vsen0is increased, or the first time verify timing is slightly delayed. Bydoing so, the PDC is controlled to be hard to become high.

FIG. 13 is a flowchart to explain a program verify operation. In FIG.13, the same reference numerals are used to designate steps havingmeaning identical to FIG. 10. As shown in FIG. 13, data is loaded, andthen, PDC data is copied into DDC. Thereafter, a program loop counterPLC counts loops PLC1, PLC2 . . . PLCn every several page or severalblock writes (S21, S22). Here, PLC1 denotes loop count in one-oldprogram, PLC2 denotes loop count in two-old program, and PLC denotesloop count in n-old program. An average value APC of these loops(PLC1+PLC2+. . . +PLCn/n) is calculated (S23). In other words, the countvalue of eh counter is divided by program counts, and thereby, theaverage value APC of the loop count is obtained. If the average valueAPC is more than a predetermined value, the level of Vsen0 in verify isincreased, or the first time verify timing is delayed (S24, S25). Inorder to delay the verify timing, the timing of generating signals VPREand BLPRE in the data storage circuit 10 may be delayed. The foregoingconfiguration is provided, and thereby, the program verify count isreduced.

There is another method of reducing the program verify count.Specifically, the control signal and control voltage generator circuit 7shown in FIG. 4 is provided with a counter, which counts the erase countevery block. Moreover, the memory cell array shown in FIG. 5 is providedwith several cells, which store the erase count every block. If theerase count stored in the cell is more than a predetermined value, thePDC is controlled to be hard to become high using the same means asdescribed above.

FIG. 14 is a flowchart to explain an erase operation. When erasing data,an erase count ELC is read from the cell storing the erase count, andthen, stored in a register (not shown) (S31, S32). The selected block iserased to verify whether or not erase is sufficient (S33, S34). Theforegoing operation is repeated until erase is completed (S35, S33,S34). If erase is completed, the erase count ELC stored in the registeris counted up (S36). Then, the counted-up erase count ELC is written inthe cell. The write operation is the same as the foregoing programoperation. Specifically, the counter PLC counting the program loop countis reset, and then, the erase count ELC is programmed in a predeterminedcell (S37, S38). Thereafter, program and verify are repeated until allPDCs becomes “1” (S39, S40, S41, S38). In the manner as described above,the erase count ELC is counted every when the memory cell is erased, andthen, stored in the memory cell.

FIG. 15 is a flowchart to explain a program operation. The program ofFIG. 15 differs from FIG. 10 in that erase count ELC stored in thememory cell is read before program (S51). If the erase count ELC is morethan a predetermined value, a detection potential Vsen0 is increased, orverify timing is delayed (S52, S53).

The foregoing configuration is given, and thereby, program time isshortened in accordance with cell degradation resulting from repetitionof program and erase.

The erase count is stored at a unit of block; however, the presentinvention is not limited to the unit of block. For example, if data iserasable at a unit of page, the erase count may be stored after iscounted at a unit of page.

The detection potential Vsen0 or the second time verify timing may bechanged regardless of cell degradation resulting from repetition ofprogram and erase. For example, a preset value may be changed every wordline or block.

Second Embodiment

According to the program verify operation of the first embodiment, thebit line of non-write cell is pre-charged to be discharged. However, ifmany non-write cells exist, current carrying the source line (SRC) islarge. For this reason, the source line becomes a floating state;therefore, the potential of the source line steps up. As a result,current does not readily flow through the write cell to be normallyverified. In view of the foregoing circumstances, according to thesecond embodiment, only bit line to be normally verified is pre-charged.

(Program)

A program operation is the same as the first embodiment. The programoperation differs from the first embodiment in that PDC data is invertedduring recovery of word line potential.

FIG. 16 shows the relationship between data stored in data cache andwrite cell. In other words, FIG. 16 shows a state after program recoveryand after PDC data is inverted.

(Inversion of PDC Data)

The following operation is carried out in order to invert PDC data.Signal VPRE=Vss and signal BLPRE=Vdd are given to obtain TDC=Vss, andthereafter, signal VREG=Vdd and signal REG=Vdd+Vth are given. IfDDC=high, TDC=high is obtained. However, if DDC=low, TDC is kept low.Signal DTG=Vdd+Vth is given to copy PDC data into DDC. Thereafter,signal BLC=Vdd+Vth is given to copy TDC data into PDC. By doing so, thePDC data is moved to DDC; on the other hand, the DDC data is moved toPDC.

Signal VPRE=Vdd and signal BLPRE=Vdd+Vth are given to obtain TDC=Vdd,and thereafter, signal VPRE=Vss and signal REG=Vdd are given. IfDDC=high, TDC=low is obtained. However, if DDC=low, TDC is kept high.

Signal DTG=Vdd+Vth is given to copy PDC data into DDC. Thereafter,signal BLC=Vdd+Vth is given to copy TDC data into PDC. By doing so, thePDC data is moved to DDC; on the other hand, the DDC data is inverted,and thereafter, moved to PDC.

(Program Verify)

A program verify operation is substantially the same as the firstembodiment. However, signal BLC=Vsg+Vth is given to obtain PDC=high; inother words, only bit line of write cell is pre-charged. Duringdischarge of bit line, data of data cache is inverted to be restored inprogram recovery before the second time verify before or after the firsttime verify. In order to invert and restore data of data cache, the dataof data cache is inverted according to the same operation as PDC data.However, if the data of data cache is inverted and restored after thefirst time verify, the DDC data is inverted. Other operation is the sameas the first embodiment.

According to the second embodiment, only bit line of the write cell tobe verified is pre-charged. Thus, current carrying the source line SRCis reduced to prevent the source line SRC from becoming a floatingstate. Therefore, current flows through the write cell to be verified,so that threshold voltage distribution is narrowed.

Third Embodiment

FIG. 17 shows an operation according to a third embodiment. In theforegoing first and second embodiments, the source line is set to Vssduring bit line discharge in verify. As seen from FIG. 17, the potentialof the source line SRC is set to Vxx (e.g., 0.2V) from program start tothe first time verify. Potential supplied to cell seemingly becomesverify potential AV-Vxx. Therefore, this implies that the same potentialas potential AVL lower than a normal verify potential AV is supplied, asshown in FIG. 1B. In this manner, the first time verify operation iscarried out, and thereafter, the potential of the source line SRC is setto ground potential, and then, the second time verify operation iscarried out.

According to the third embodiment, the capacity of the source line issmaller than that of the word line; therefore, a high-speed verifyoperation is possible.

In the third embodiment, the potential of source line only is set to Vxx(e.g., 0.2V), and thereby, the threshold voltage of cell slightly stepsup according to back bias effect. In this case, the potential of well(not shown) formed with memory cell is set to Vxx (e.g., 0.2V), andthereby, it is possible to prevent a step-up of the threshold voltage ofcell.

Fourth Embodiment

In the first to third embodiments, binary data is stored in the memorycell. The present invention is applicable to a multi-valued memorystoring several bits in one cell.

FIG. 18 shows the configuration of a data storage circuit 10 applied tothe case of storing ternary data. The data storage circuit 10 shown inFIG. 18 differs from that shown in FIG. 8 in the following point.Specifically, the data storage circuit 10 is further provided with SDC(secondary data cache), transistors 61 n, 61 o as a transfer gate fortransferring a signal COMi, and transistor 61 g.

Each row of the memory cell array 1 shown in FIG. 5 is provided withflag cells (not shown). The bit lie connected to the flag cells isconnected to a flag cell data storage circuit. When second page data iswritten, each flag cell is stored with data showing that the second pagedata is written.

FIG. 19 shows the sequence for programming ternary data, and FIG. 20shows a change of the threshold voltage of memory cell according to aprogram operation.

As depicted in FIG. 20, if ternary data is written, the cell thresholdvoltage is transferred from an erase state to level “A” or “B”. For thisreason, according to the fourth embodiment, verify at a verify level “A”(S14) and verify at a verify level “B” (S60) are carried out, as seenfrom FIG. 19. The verify at a verify level “B” is the same as the firstto third embodiments. However, if verify of the cell written at thelevel “A” is carried out in the same manner as verify of the cellwritten at the level “B”, the following problem arises. That is, a cell,which is written at the level “B” and does not reach the “B” level,passes the verify. Thus, if write at the level “B” is carried out duringa write sequence, a node N2 b of the SDC in the data storage circuit 10shown in FIG. 18 is set high. On the other hand, if write at the level“A” is carried out, the node N2 b is set low. In verify at the verifylevel “A”, when the bit line potential is captured in TDC, the TDCbecomes high. This is the case where the threshold voltage is higherthan the level “A”. For this reason, a signal BLC2 is set to Vth+0.1V sothat TDC is forced low. By doing so, even if write at the level “B” iscarried out, the TDC is set low not to pass the verify.

According to the fourth embodiment, it is possible to carry out theverify operation at high speed even if multi-valued data is stored.

Fifth Embodiment

According to the foregoing embodiments, the write speed is made late inthe following manner in the program verify read operation. Specifically,as shown in FIG. 21, when the verify level exceeds a low verify level a*or current Icell flowing through cell is a little, an intermediatepotential (e.g., 1V) is supplied to the bit line to make the write speedlate.

On the contrary, according to the fourth embodiment, the write speed ismade late in the following manner in the program verify read operation.Specifically, as shown in FIG. 22, when the verify level exceeds a lowverify level a** or current Icell flowing through cell is a little, anintermediate potential (e.g., 1.2V) is supplied to the bit line to makethe write speed late. Moreover, when the verify level exceeds a lowverify level a* or current Icell flowing through cell is less thanabove, an intermediate potential (e.g., 0.4V) is supplied to the bitline to make the write speed later than above, and then, write isrepeated. By doing so, threshold voltage distribution is narrowed.

FIG. 23 shows the configuration of a data storage circuit 10 applied tothe fifth embodiment. When the verify level exceeds verify level a**, a*and write or non-write state, data is stored in each of DDC, DDC2 andPDC.

In the fifth embodiment, two levels a** and a* are given; in this case,several levels may be given.

According to the fifth embodiment, the intermediate potential suppliedto the bit line in a verify operation is increased. By doing so, thewrite speed is properly set, and threshold voltage distribution writtento cell is narrowed. Moreover, the write speed is made late usingseveral intermediate voltages; however, write time at one time is not soincreased. Therefore, there is no hindrance to a high-speed programverify operation.

Sixth Embodiment

In FIG. 1B, the potential of the word line WL is set to a potential AVLlower than a normal verify potential AV in verify, and thereafter, setto the normal verify potential AV. However, if the verify potential setas described above, the following problem arises. Specifically, inLog(I)-V characteristic, characteristic C2 having a gradient smallerthan characteristic C1 is given resulting from reduction of cell currentas shown in FIG. 24. In this case, current is carried even if the verifypotential AVL is given. For this reason, the cell threshold voltagebecomes higher than the threshold voltage (AV) of a write target.

In the foregoing first to fifth embodiments, the word line potential isunchanged in verify, and thereby, word line potentials in verify andread are made equal. By doing so, margin in read is reduced. In otherwords, the word line potential in verify is aligned with that in read.

On the contrary, according to the sixth embodiment, as shown in FIG. 25,the word line potential is changed in a read operation in the samemanner as the verify operation shown in FIG. 1B. For example, thepotential of the word line WL is set to a level (ARL) lower than anactual read level to discharge a current. Thereafter, the potential ofthe word line WL is set to an actual read level (AR). The foregoingoperation is given, and thereby, read and verify operations are carriedout under the same condition. Therefore, there is no need of wideningmargin in data read any more, and data stored in the memory cell issecurely read.

Seventh Embodiment

FIG. 26A, FIG. 26B and FIG. 26C show a write sequence of storing 2-bitdata in one memory cell using quaternary (four-valued) data. FIG. 27 andFIG. 28 each show a flowchart to explain a quaternary (four-valued)write operation of changing a verify level. FIG. 27 shows a flowchart toexplain a first page program operation, and FIG. 28 shows a flowchart toexplain a second page program operation.

As shown in FIG. 26A, when an erase operation is carried out, memorycell data becomes “0”. According to a first page write, the memory celldata is kept at “0” if data from the outside is non-write “1” whilebecoming “1” if data from the outside is write “0”.

As depicted in FIG. 26C, in the second page write, the memory cell datais kept at “0” if the memory cell data is “0” and data from the outsideis non-write “1”. If data from the outside is write “0”, the memory celldata becomes “2”. If the memory cell data is “1” and data from theoutside is non-write “1”, the memory cell data becomes “4”. If data fromthe outside is write “0”, the memory cell data becomes “3”.

In the first page write, threshold voltage distribution width may belarge. Thus, as described in FIG. 24, in Log(I)-V characteristic,characteristic C2 having a gradient smaller than characteristic C1 isgiven resulting from reduction of cell current. For this reason, thecell threshold voltage becomes higher than the threshold voltage of awrite target. But, this is no problem. However, in the second write, thethreshold voltage distribution width must be narrowed. Specifically, asdescribed in FIG. 24, in Log(I)-V characteristic, characteristic C2having a gradient smaller than characteristic C1 is given resulting fromreduction of cell current. As a result, cell carries current at theverify potential AVL. For this reason, the cell threshold voltagebecomes higher than a write target the threshold voltage (AV), andthereby, the threshold voltage distribution width is widened. Thisresults from the reason why the difference between verify potentials AVLand AV is narrow.

According to the second page program operation shown in FIG. 28, thefirst time verify is carried out in the following manner. Specifically,in verify (AVL/AV) (S70), the bit line is pre-charged, andsimultaneously, the word line potential is set to a level AVL lower thana normal verify level AV, as shown in FIG. 1B. Thereafter, the word linepotential is set to a normal verify level AV, and then, the second timeverify is carried out. In verify (BVL/BV)(S71), verify is carried out ata level BVL lower than a normal verify level BV and at the normal verifylevel AV, like the foregoing verify (AVL/AV).

On the contrary, according to the seventh embodiment, verify is carriedout in the following manner. According to the second page program shownin FIG. 29 and FIG. 30, in verify (AVL/BVL) (S80), only verify at averify level lower than a normal verify level AV/BV is carried out.

First, the bit line is pre-charged, and simultaneously, the word linepotential is set to AVL to carry out a first time verify operation at alevel A is carried out. Thereafter, the word line potential is changedfrom AVL to BVL to carry out a second time verify operation a level B.

In verify (AV/BV) (S81), a verify operation is carried out at a normalverify level (AV/BV). Specifically, the bit line is pre-charged, andsimultaneously, the word line potential is set to a level AV to carryout a first time verify operation at a level A. Thereafter, the wordline potential is changed from AV to BV to carry out a second timeverify operation at a level B. In this case, the potential differencebetween verify levels AVL and BVL and AV and between BV is large;therefore, no problem described in FIG. 24 arises. Thus, the thresholdvoltage distribution is narrowed, and high-speed write is achieved.

FIG. 31 and FIG. 32 each show a flowchart to explain a read operation ofchanging a verify level. FIG. 31 shows a first page read operation, andFIG. 32 shows a second page read operation.

According to the first page read operation shown in FIG. 31, data isread at a read level BR (S91). The read result is transferred from PDCto SDC (S92). Thereafter, flag cell data (written only when second pagewrite is carried out) for identifying whether or not second page writeis made is determined (S93). If data is written in the flag cell, thedata is outputted (S94). On the other hand, if data is not written inthe flag cell (no write is made with respect to second page), data isagain read at a read level AR to output data outside (S95, S96, S94).

On the contrary, according to a first page read operation of the seventhembodiment, a read operation at read levels AR and BR is carried out inthe same manner as the verify operation, as shown in FIGS. 33 and 34.Specifically, the bit line is pre-charged one time, and thereafter, theword line potential is set to AR to read data, and thereafter, the wordline potential is again set to BR to read data (S111). By doing so, theoperation in verify and read is carried out under the same condition.Thus, there is no need of widening data read margin any more, and datastored in the memory cell is securely read. In this case, the readresult at the read level AR is held in the DDC; on the other hand, theread result at the read level BR is held in the PDC.

Thereafter, data is transferred from PDC to SDC (S112). Flag cell foridentifying whether or not the second page write is carried out isdetermined (S113). As a result, if the flag cell is written, the data isoutput (S114). On the other hand, if data is not written in the flagcell (no write is made with respect to the second page), the read resultat the read level AR held in the DDC is transferred to SDC (S115) tooutput data outside (S114).

According to the seventh embodiment, the following advantage is given.That is, if no second page write is carried out in the first page read,two-time read sequences are required; on the contrary, one-time readsequence is only carried out. Therefore, a high-speed read is achieved.

In the first page read operation, the second page read sequence is thesame in cases where the word line potential is changed or not changedafter one-time pre-charge. Specifically, as shown in FIG. 32, the bitline is pre-charged, and thereafter, the word line potential is set toCR to read data from memory cell (S101). Thereafter, the bit line ispre-charged, and thereafter, the word line potential is set to AR toread data from memory cell (S102). Then, PDC data is transferred to SDC(S103). Thereafter, flag cell data is determined (S104). As a result, ifno write is made with respect to the second page, data is fixed to “1”,and then output (S105). On the other hand, if write is made with respectto the second page, SDC data is output (S106).

FIG. 35 shows a modification example of the seventh embodiment. FIG. 35shows an operation of continuously reading four levels after a bit lineis pre-charged. Specifically, the bit line BL is pre-charged, andthereafter, the word line WL is successively supplied with severaldifferent low verify voltages AVL, BVL, CVL and DVL. These verifyvoltages AVL, BVL, CVL and DVL are supplied to continuously carry outlow first to fourth verify. Thereafter, the bit line BL is againpre-charged, and thereafter, the word line WL is successively suppliedwith several different normal verify voltages AV, BV, CV and DV. Thesenormal verify voltages AV, BV, CV and DV are supplied to continuouslycarry out normal first to fourth verify.

According to the foregoing modification example, four-time verify iscarried out with respect to one-time bit line pre-charge. Therefore, thenumber of pre-charging the bit line is reduced, and four-valued datawrite is achieved at high speed.

Eighth Embodiment

FIG. 36 and FIG. 37 each show an eighth embodiment modifying the seventhembodiment. In FIG. 36, the same reference numerals are used todesignate portions identical to FIG. 33. In FIG. 37, the same referencenumerals are used to designate portions identical to FIG. 32.

According to the eighth embodiment, the memory cell array is furtherprovided with another flag cell. It is recognizable whether failureoccurs in page using data stored in the flag cell. Specifically, eachrow of the memory cell array 1 shown in FIG. 5 is provided with a flagcell (not shown). A bit line connected to the flag cell is connectedwith a flag cell data storage circuit. The foregoing configuration isgiven, and thereby, if failure occurs in page, data is written in theflag cell corresponding to the failure page. If failure occurs in firstpage write, data showing failure page corresponding to the first page,for example, “0” is written in the flag cell. Moreover, failure occursin second page write, data showing failure page corresponding to thesecond page, for example, “0” is written in the flag cell.

In the first page read shown in FIG. 36, it is determined whether or notdata “0” showing failure page is written in the first page of the flagcell (S121). As a result, if no data showing failure page is written,data is output as described before. On the other hand, if data showingfailure page is written, all output data of the first page are fixed to“0” (S122). By doing so, it is recognizable that the first page isfailure page.

In the second page read shown in FIG. 37, it is determined whether ornot data “0” showing failure page is written in the second page of theflag cell (S131). As a result, if no data showing failure page iswritten, data is output as described before. On the other hand, if datashowing failure page is written, all output data of the first page arefixed to “0” (S132). By doing so, it is recognizable that the first pageis failure page.

The flag for identifying failure page is set independently from firstand second pages. The first and second pages are the same cell. For thisreason, for example, if the first page is failure, there is the casewhere the second page is also failure. Therefore, only one flag foridentifying failure page is set. If write is made with respect to theflag, output data is fixed in both first and second page.

In the foregoing fifth to seventh embodiments, the bit line potential isread in the following manner. Specifically, as shown in FIG. 1B, the bitline is one pre-charged to rise up the word line, and thereafter, theselect gate is powered on to carry out the first time read of the bitline potential. Then, the word line potential is changed to carry outthe second time read of the bit line potential.

On the other hand, as depicted in FIG. 38, the bit line BL is onepre-charged to rise up the word line to a potential AVL. Thereafter, theselect gate SG is powered on to carry out the first time read of the bitline potential. Then, the select gate SG is powered off to change theword line potential to AV, and thereafter, the select gate SG is againpowered on to carry out the second time read of the bit line potential.By doing so, in the word line having large capacity and resistance, therise speed of the word line serves to prevent the difference ofdischarge speed of the bit line resulting from the difference betweenproximal and tip ends of the word line.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A semiconductor memory device comprising: a memory cell array havingmemory cells arrayed in a matrix, which are connected to a word line anda bit line and store n values (n: natural number more than 3); and acontrol circuit which controls each potential of the word line and thebit line in accordance with input data, and writes data to the memorycells, the control circuit carrying out a write operation to write athreshold voltage of k value (k≦n) to the memory cell, and charging thebit line through an N channel MOS transistor having one end connected tothe bit line and a gate set at an intermediate potential, and thereafterchanging the word line potential m times and verifying whether or notthe memory cell reaches a target m threshold value (m≦k) by detecting apotential of another end of the N channel MOS transistor at eachpotential of the word line.
 2. The device according to claim 1, whereinwhen reading data of j value (j≦n), the control circuit charges the bitline through the N channel MOS transistor having the gate set at theintermediate potential, and changes the word line potential m times, andthereafter carries out a read operation by detecting a potential of saidanother end of the N channel MOS transistor at each potential of theword line.
 3. A semiconductor memory device comprising: a memory cellarray having several memory cells arrayed like a matrix, which areconnected to a word line and a bit line and store n values (n: naturalnumber more than 3); and a control circuit which controls each potentialof the word line and the bit line in accordance with input data, andwrites data to the memory cell, the control circuit carrying out a writeoperation to write a threshold voltage of k value (k≦n) to the memorycell, charging the bit line through an N channel MOS transistor havingone end connected to the bit line and a gate set at an intermediatepotential, and thereafter changing the word line potential m times andverifying whether or not the memory cell reaches an m threshold value(m≦k) lower than a target threshold voltage by detecting a potential ofanother end of the N channel MOS transistor at each potential of theword line, charging the bit line through an N channel MOS transistorhaving a gate set at an intermediate potential, and thereafter changingthe word line potential m times and verifying whether or not the memorycell reaches a target m threshold value (m≦k) by detecting the potentialof said another end of the N channel MOS transistor at each potential ofthe word line, and reducing write speed when the threshold voltage ofthe memory cell has reached a threshold voltage lower than the target mthreshold voltage, and carrying out no write operation when thethreshold voltage of the memory cell has reached the target m thresholdvoltage in next write operation.
 4. The device according to claim 3,wherein when reading data of j value (j≦n), the control circuit chargesthe bit line through the N channel MOS transistor having the gate set atthe intermediate potential, and changes the word line potential m times,and thereafter carries out a read operation by detecting a potential ofsaid another end of the N channel MOS transistor at each potential ofthe word line.